If a paper web is to be well formed a good grammage distribution must be achieved, by which is meant a uniform distribution of fibers in the paper. Good formation improves drainage, pressing and drying and makes creping less prone to problems, while at the same time the paper becomes stronger and softer. The function of a headbox is thus to distribute the fiber suspension, the stock, in such a manner that said good formation is obtained. This is achieved by a jet of stock being delivered from the headbox through a stock outlet, the discharge opening, formed by upper and lower lips, so that the jet of stock encounters a forming wire in a forming zone. The paper web is then formed by water being drained out through the wire so that the layer of stock is gradually thickened to a continuous fiber network. To obtain a correct sheet structure with varying jet thickness, web speed or type of stock it is therefore of great significance that the stock is delivered in a manner carefully determined for a particular production since the drainage, and thus the formation, is greatly affected by where and how the stock jet encounters the wire, i.e., the angle of impact, the distance from the discharge outlet and the speed at which the stock jet encounters the forming wire.
To enable desired alteration of the point of encounter of the stock jet, its length and angle of impact, the headbox and its stock outlet must be adjustable. The thickness of the stock jet is regulated by means of setting devices which set the geometry of the lip opening. When the headbox is installed after exchanging, service, etc., it must be set in a position and aligned in relation to the forming zone so that said three parameters are correct.
The headboxes of soft paper machines do not differ in basic principle from other boxes, the box type depending on the speed range. Even at a low speed, i.e., above 300 m/min., closed hydraulic headboxes are used nowadays, which give a more stable profile and better formation. The speed into the hydraulic headbox is chosen so that good fiber distribution is obtained, and suitable turbulence intensity in the stock without unnecessary pressure losses. However, this requires the hydraulic headbox to be run with the flow for which it was dimensioned since a lower flow rate will give poorer fiber distribution and a poorer grammage profile. If the hydraulic headbox must be changed because a new optimal flow requires it, it must be possible to adjust the stock jet quickly and simply to a new position for the new hydraulic headbox.
Twin wire machines in which dewatering occurs between the twin wires have been known since the 70s. In essence this type of wire machine comprises a breast roll and a forming roll which rolls, together with said wires, form an inlet nip through which both wires then run together along almost 180.degree. of the circumference of the forming roll, forming outer and inner wires. Only hydraulic headboxes are used as headboxes for twin wire machines and these hydraulic headboxes must be constructed so that they can be inserted into the nip between the wires since the free stock jet should not exceed a maximum length of about 250 mm, preferably about 100 mm. This is important so that the jet, which may have a speed of 1800-2000 m/min., is not broken up by turbulence before it encounters the forming wire. Even after deaeration, the stock jet contains undesired pressurized air bubbles which increase the risk of said turbulence arising, particularly at high machine speeds, since the air bubbles in the stock jet quickly expand when they emerge from the hydraulic headbox, thus disintegrating and deforming said jet. The lips are of approximately equal length, which contributes to facilitating adjustment of the stock outlet when the position of the headbox must be adjusted or re-set. Even so, said adjustment constitutes extremely complicated and time-consuming work due to the lack of space in said inlet nip and the hydraulic headbox must therefore first be adjusted roughly before any fine adjustment of the discharge opening can be carried out with the aid of setting devices.
The hydraulic headbox is placed close to the forming roll, just before the inlet nip, with its angle of impact and alignment carefully set. Since a very large part of the dewatering process occurs in the inlet nip, what happens there is extremely important to the forming of the paper.
The best formation is obtained if about 30-40% of the stock jet lies below the tangent to the forming roll in the direction of the jet so that part of the stock jet encounters the forming roll. This results in a pressure surge at the impact point which can, however, be regulated by lifting the jet so that its lower surface only touches the forming roll at a tangent. The more of the stock jet lying below the tangent to the forming roll the greater will be the pressure surge because more of the jet encounters said forming roll. The best position for the stock jet, and thus for the stock jet at different web speeds, stock concentrations and grades of paper with different requirements as to what constitutes an acceptable formation, must therefore be determined by trial and error. The setting device for the hydraulic headbox must also be so stable that the relation of the stock jet to the forming zone is not altered.
Several designs of twin wire machines are currently available, which differ from each other with regard to location of the hydraulic headbox, inclination of the nip, etc.
Depending on the design of the wet section, the stock jet can be directed downwardly, horizontally or upwardly. The wire mesh contains air which must be removed. An upwardly directed jet gives the best deaeration and the least risk of liquid splashes, but also has the drawback of more complicated dewatering. If, instead, the stock jet encounters the forming wire in an inclined, downwardly directed nip, while the wires are running upwardly over the forming roll, the dewatering takes place substantially downwards. All three alternatives are used with success.
If the stock jet is directed towards a desired point of impact it must be re-set or adjusted when trimming or regrinding the roll, after a roll exchange or wire replacement, etc. When replacing the wire, for instance, the headbox must be moved and with the bulky adjustment devices available today, this takes far too long. The hydraulic headbox must also be accessible for inspection and cleaning and must therefore be easily accessible so that it can be opened. A simple adjustment device, allowing all the above-mentioned configurations to be set, as well as efficient and quicker resetting after roll or wire replacement would be extremely profitable.
The machine manufacturers also at present build hydraulic headboxes capable of spraying two or three stock jets simultaneously into the nip between two wires, the individual jets being kept apart with the aid of wedges of air right up to the point of impact, and the length of the stock jets, i.e., the setting of the distance between the hydraulic headbox and the inlet nip, acquires increased significance.
The hydrodynamics become extremely important at the high speeds applicable for modem soft paper machines. Considerable demands are placed, for instance, on the hydraulic headbox being able to supply a stock jet having such well suppressed turbulence that it stays together until it reaches the nip between the wires. The direction and point of impact in this nip are then critical. Due to the cramped space between the inlet nip and the hydraulic headbox, adjustment of a new angle of impact while retaining the length of the jet may be extremely complicated. With the link system used today these small changes constitute a great problem since such small calibrations are not easy to perform without considerable risk of the hydraulic headbox coming into contact with and damaging the other equipment during this adjustment.
As mentioned above, a major part of the dewatering takes place in the inlet nip and it is thus of the utmost importance that the dewatering is correctly set here. This initial dewatering is directly depended on the angle of impact of the stock jet on the wire. Good formation is obtained if the fibers can be kept dispersed in the free stock layer until they are completely fixed in the growing fiber network without forming fiber flocks before then. This process is affected by the sort of stock being used at the time. Stock of different origin, such as hardwood or softwood, additives, temperature, etc. have different forming properties and it must therefore be possible to set different angles of impact and jet lengths depending on the current properties, and this must also be carried out quickly and cost-effectively since all standstills in production are extremely expensive. If dewatering takes place too slowly the turbulence in the stock layer will decrease and the risk of fiber flocks forming increases. If dewatering takes place too quickly, due to too sharp an angle of impact for instance, this will lead to the fiber network becoming fixed too early. The fiber bed will then be too dense and fibers can easily be drawn in and get caught in the meshes of the wire. This damages the paper and also clogs the wire. It must therefore be possible to set the angle of impact of the stock jet with extremely high precision, which is impossible with the bulky constructions available today. These constructions are simply unable to perform small alterations in a quick, stable and simple manner.
Generating turbulence in the stock layer during the active dewatering process which takes place for the most part immediately after encountering the wire and during passage of the wire through the inlet nip, disintegrates any fiber flocks that may have formed and prevents the occurrence of new ones, thereby improving the formation of the paper. The angle of impact of the jet, i.e., how much of the stock jet encounters the forming roll, with resultant variation of turbulence and stock pressure, has the most dominant effect on successful dewatering and thus also on the formation. Other factors affected by angle of impact are splashing water and difficulties removing the waste water.
Said adjustment of the headboxes has hitherto been performed with the aid of several different adjustment devices all of which, however, are extremely expensive. Canadian patent specification CA-1,098,748 describes a setting device comprising a link system consisting of a number of link arms operated by several actuators in a complicated manner. Furthermore, said link system supports the entire weight of the complete headbox construction and the construction elements are therefore extremely robust and expensive. Actuators in the form of light, small hydraulic or pneumatic piston type cylinders cannot be used since if a pressure drop were to arise due to the pivotable suspension this would immediately result in an altered angle of impact for the stock jet, and consequently deteriorated formation. Furthermore, since all movements in the described suspension arrangement consist of turning movements, the point of impact cannot be altered without simultaneously altering both the horizontal and the vertical position. Neither is it possible to maintain a predetermined length of the stock jet. With the heavy and ungainly constructions used here it is also completely impossible to obtain definite and exact movements with the aid of only link arms and actuators. The headbox must therefore be manipulated to and fro repeatedly before a more or less acceptable position can be obtained.
A headbox is also known through the published patent application SE-447 139 which is supported by forward and rear stand constructions on which horizontal sliding guides are arranged. The headbox can be displaced in a horizontal direction with the aid of a first actuator. Another actuator is arranged at the rear of the headbox which also enables turning about a shaft at the forward stand construction. The headbox can thus be displaced in a horizontal direction, which influences the jet length for instance, and can be turned about said forward shaft so that the angle of impact and point of encounter can be varied. However, the headbox cannot be moved vertically and this setting device is thus clearly restricted in its adjustment ability in relation to a fixed inlet nip.
German published patent application DE 43 28 997, finally, describes a setting device in which an attempt is made to manage said complex adjustment of both a desired direction and a specific point of encounter, while at the same time maintaining a predetermined stock jet length. The entire weight of both headbox and setting device is carried by the described actuator and its journalling means. It will be readily understood that such a setting device, which must be dimensioned to deal with extremely heavy loads, approximately 40 ton, requires an extremely strong and expensive construction. Since the center of gravity of the construction will vary depending on the location of the headbox, each element in the construction e.g., stock, actuator, etc., must be dimensioned for a plurality of movable and variable load cases where the weight of the entire adjusting device, including the headbox, in an extreme case is supported almost entirely exclusively by one of the elements, see FIG. 3 in the publication described here. The elements used will therefore be extremely over-dimensioned in comparison with if the loads were distributed more uniformly between part-elements. Problems therefore occur in fine-adjustment of the stock jet since the actuator cannot provide the accuracy and flexibility required. The setting device shown comprises at least three pairs of rigidly mounted jacks which make the construction more expensive and, due to the considerable loads, are subjected to considerable wear. Since the construction is suspended in a system of actuators, all of which are free to move in relation to each other, it is also difficult to suppress the unavoidable vibrations in a satisfactory manner.
A main object of the present invention is at least for the most part to eliminate the problems mentioned above and achieve an improved device for setting the position of a headbox so that its stock jet can be quickly, simply and reliably set in an optimal position in relation to its point of encounter, length and angle of impact in relation to a forming zone at the inlet nip to a twin wire machine.
Another object of the invention is to provide a device for setting the position of a headbox so that a paper web with good formation can be produced regardless of the type of stock or the rate of flow deemed optimal for the headbox design or wire configuration concerned.
Yet another object of the invention is to provide a device for setting a headbox so that all vibration and instability in the headbox is eliminated or at least greatly reduced.
Still another object of the invention is to provide a device for setting a headbox depending on the paper grade, pulp quality or machine speed, so that its alignment and distance can be adjusted as to height, length and inclination in a considerably simpler and more flexible manner, at a third of the normal cost.